The biggest challenge in optical telecommunications today is increasing the capacity of installed fibre links to maintain the year-on-year growth in demand for bandwidth. It has been acknowledged within the industry that transitioning to high spectral efficiency coherent modulation formats such as quadrature phase-shift keying (QPSK), quadrature amplitude modulation (QAM) and the like is important, and 100 Gbit/s QPSK has been deployed on several long haul links worldwide. These modulation formats include modulation in phase, and thus the use of low-cost direct modulation of the signalling laser is not possible due to the high chirp associated with direct modulation of a semiconductor laser, which significantly limits transmission distance.
Today, the most robust solution for generating complex modulation formats is the LiNbO3-based external IQ modulator. However, its drawbacks include: (i) relatively high cost; (ii) significant insertion loss and limited optical power handling; (iii) a need for high drive voltage RF booster amplifiers; and (iv) for generation of complex modulation formats (e.g., 16 QAM), a typical IQ modulator requires two or more high-speed multiple-level (e.g. four logic levels for 16 QAM) data streams to be generated and multiplexed in the electronic domain with associated loss, power consumption, noise and non-linearity.